Large-sized forged steels (steel forgings) have satisfactory strength and toughness, have been regarded as suitable assembly members for pressure vessels, steam generators, and other equipment in nuclear power facilities, and have been widely used as members for components for nuclear power plants. Nuclear power generation has recently been employed more and more, because this technique is free from carbon dioxide (CO2) emission and is advantageous from the viewpoints of protection in global environment and particularly prevention in global warming. In addition, more and more energy has recently been required than ever before, and this demands further larger-sized pressure vessels, steam generators, and other equipment in nuclear power facilities.
Pressure vessels, steam generators, and other equipment in nuclear power plants have had larger and larger sizes as described above. This requires large-sized forged steels for use therein to have further satisfactory strength and toughness and to exhibit satisfactory hydrogen cracking resistance (hydrogen embrittlement resistance).
Large-sized forged steels may be used as base materials (base metals) to assemble welded structures for components for nuclear power plants. Such structures after welding are generally subjected to a long-term stress relief heat treatment in order to relieve the stress. The large-sized forged steels for components for nuclear power plants should have strength and toughness at satisfactory levels even after the stress relief heat treatment.
The large-sized forged steels for components for nuclear power plants should have strength, toughness, and hydrogen cracking resistance at satisfactory levels as above. Techniques relating to steels having satisfactory strength and toughness have been proposed from long ago as in Patent Literature (PTL) 1 to 4. Nuclear power plants in those times, however, were designed before the upsizing and required strength and toughness at not so high levels as compared to those of current large-sized equipment.
Improvements in hydrogen cracking resistance have been studied both from steel refining techniques and from steel chemical compositions and structures. In view of the refining techniques, real operations have already employed a specific technique. In this epoxy resin composition, the upper limit of a hydrogen level upon molten steel refining is specified, and a hydrogen gas removing treatment is performed when an actual hydrogen level exceeds the specified upper limit. The hydrogen gas removing treatment is believed to have a ceiling in hydrogen reduction from the viewpoints of treatment time and cost. For these reasons, current production of forged steels employs control of hydrogen on the order of from one to several parts per million. The current hydrogen control on the order of from one to several parts per million, however, fails to completely prevent hydrogen cracking because hydrogen cracking is caused by hydrogen at a lower level than this.
From the viewpoints of steel chemical compositions and structures, PTL 5 has proposed a technique for improving hydrogen cracking resistance as a method for refining a molten steel. In this technique, MnS inclusions are positively introduced into the steel by increasing a S content in the steel, and the MnS inclusions are effectively used as diffusible-hydrogen trapping sites. Although certainly improving hydrogen cracking resistance, even this technique hardly prevents hydrogen cracking completely. This technique disadvantageously causes deterioration in toughness due to inclusions in larger amounts, although it provides somewhat better hydrogen cracking resistance.